1. Field of the Invention
This invention relates to fluid supplies for electrostatic spray systems and more particularly, relates to a grounded insulated fluid supply.
2. Background Information
A recent increase in the use of waterborn coatings, sprayed electrostatically, has created interest in providing protection against shock hazards. Previously, when spraying waterborn coating electrostatically, the supply tank for the coating, which is normally quite conductive, is isolated from ground with electrical insulators. This is achieved by supporting the supply tank on electrical insulators of sufficient insulation values to assure high voltage is maintained to the electrostatic spray device. In these systems the support tank, as well as the supporting platform, are carried at high voltage which can create shock hazards.
To provide safety for personnel operating such systems, the supply is then often enclosed in a grounded and fenced area that typically has a gate opening that is interlocked so that the supply is automatically grounded if the gate is open. This arrangement requires substantial additional floor space in the spray area necessitating high initial costs for materials, installation and maintenance. Also, the system must frequently be checked to ensure proper operation.
Operation of electrostatic spraying systems typically range in voltage outputs from 60,000 to 100,000 volts DC depending upon the type, and whether they are manual or automatic. The electrical capacitance of such operating systems can be quite high and undesirable.
These problems have generated substantial interest in making the system more simple and safe by using voltage blocking systems. These voltage blocking systems provide protection from shock hazards which present serious problems because waterborn coatings are such good conductors of electricity. The charge applied to these coatings feeds back through the paint stream and electrifies the paint equipment. The most common safety strategy, as described above, is to place the paint supply equipment on insulated stands and isolate them from ground. Building safety cages around the equipment so no one is able to come into contact while it is charged is another step toward safety.
In recent years, paint equipment and other fluids spray suppliers have developed voltage blockers to cope with this problem. Each use different designs and concepts that vary, but all seem to achieve the same goal which is to create an air gap in the fluid stream to prevent the flow of electricity between electrostatic spray device and the paint supply.
One such voltage blocker called the Side Kick Voltage Blocker, manufactured by Binks Manufacturing, Inc., offers a computer-controlled voltage block. The Position between the spray device and the grounded material supply are isolated to progressively block the electrostatically charged reservoir or supply tank from ground. This system provides protection by moving a fixed volume of waterborn paint from isolator to isolator and finally into the electrostatically charged reservoir. Each isolator tube is scraped and cleaned as it transfers paint, thereby creating a voltage block. The system continuously fills and maintains a fluid level in the reservoir as paint is being sprayed.
Another system, manufactured by Graco Inc. of Minneapolis, Minn. called the H.sub.2 O Pro Voltage Blocker, uses voltage blocking designed around an isolator that supplies paint to an accumulator. The isolator houses a valve that alternately opens to allow paint to fill the accumulator and then closes to provide electrical separation between charged fluid and grounded equipment. The system also features an arc reduction probe on the isolator that protects the wiping rod from arcing and a bleed resistor that drains voltage from the system when the gun is not use. The system also includes a pump that circulates lubricant seals in the isolator, a push-button flushing activator and a grounded metal enclosure that eliminates static discharge.
Another system called the Aqua Block System, available from ITW Ransburg Electrostatic Systems of Toledo, Ohio is comprised of three basic elements. These elements are a rotary four-way purging valve, a cylinder/reservoir and a circulating supply of "blocking media." The fluid supplied to the spraying apparatus enters one port of the four-way valve and is alternately switched from one or the other of the valve to the port at each end of the cylinder/reservoir. The cylinder/reservoir is a double-ended piston assembly that is driven by the flow of fluid through the system to the spraying equipment. The continuous flow of blocking medium purges any conductive fluids that escape as the valve actuates, providing electrical isolation inside the four-way valve.
Another system called the Iso-Flo Voltage Blocker from Nordson Corporation of Amherst, Ohio employs voltage blocking designed to work with nearly all internal and external power supply guns, atomizers and discs. In this system, an electrostatic power unit is located just outside the spray booth to supply the electrostatic charge to the coating material. This charging method eliminates the need for individual power supplies and electrostatic cables from multiple spray devices. The system features single-stage, pneumatically operated reservoirs that alternately fill and spray to ensure a continuous supply of coating material. During operation, shuttle valves alternately connect the reservoirs to the grounded paint supply source and to the charge spray devices. The shuttle valves provide the air gap to provide the voltage blocking or electrical isolation between the charged coating and any grounded material or equipment.
While the above systems may be effective, they are complicated and expensive to fill and use, requiring a great deal of maintenance and many parts. It would be advantageous if a system could be designed and operated effectively in which the supply tank is grounded and insulated for use with a conductive coating contained within the tank. Such a system would void all the disadvantages of electrostatic application of waterborn coatings and those of the present complex isolation systems described above.
It is therefore, one object of the present invention to provide a grounded insulated supply tank for electrostatic application of waterborn coatings.
Still another object of the present invention is to provide a grounded insulated supply tank for use in electrostatic application of waterborn coatings that will minimize the amount of floor space required.
Yet another object of the present invention is to provide a grounded insulated fluid supply that is simple and easy to use, and substantially reduces installation costs.
Still another object of the present invention is to provide a grounded insulated supply tank for electrostatic application of waterborn coatings that has a conductive tank with a insulated liner and a non-conductive pick-up tube that is attached to the fluid output of the supply tank cover and extends to the bottom of the inner most insulating liner.
Still another object of the present invention is to provide a grounded insulated supply tank for use in electrostatic application of waterborn coatings including a non-conductive fluid hose connected to an insulating fluid pick-up tube in the fluid tank. The non-conductive fluid hose may have a conductive outer cover to ground the system in the event of hose rupture and offers abrasion protection in use.
Still another object of the present invention is to provide a grounded insulated supply tank for electrostatic application of waterborn coatings that has multiple insulating liners, an insulated cover on top of the insulating liners in addition to an insulating pick-up tube that isolates the electrically conductive waterborn coatings from the conductive tank, allowing the tank to be grounded.
Yet another object of the present invention is to provide grounded insulated supply tank for electrostatic application of waterborn coatings that includes a grounding probe to ground the electrically conductive fluid when fluid in the tank is depleted.
Still another object of the present invention is to provide a grounded insulated supply tank for electrostatic application of waterborn coatings that include an automatic grounding system when the tank is empty and tank pressure is released.
Still another object of the present invention is to provide a grounded insulated supply tank for electrostatic application of waterborn coatings having a grounding system including a piston that is lifted when the tank is pressurized and is released by removing pressure when the tank is empty to automatically ground the electrically conductive fluid.